Various authentication techniques are used to restrict access to electronic devices and ensure that only authorized users may gain access to sensitive resources. For example, biometrics may be used as a convenient and reasonably secure basis for authenticating users. Biometric signatures, such as fingerprints or iris images, are acquired from the user's person, meaning a user authenticated through biometrics would not need to remember cumbersome passwords or the like. However, biometric signatures are vulnerable to spoofing. Fingerprints, for example, can be duplicated with off-the-shelf materials, while many iris scanners can be fooled with simple high-resolution photographs. Because most biometric signatures by their very nature cannot be changed, users who have had one or more of their biometric signatures compromised may perpetually remain susceptible to security breaches. Accordingly, a need exists for advanced authentication techniques that avoid some of the disadvantages of conventional authentication techniques.